1. Field of the Invention
The present invention relates to the art of gas analysis, and more particularly, to a process and apparatus for the removal of moisture from gases to produce a sufficiently dry sample for accurate measurement of constituent elements in the gases.
2. Description of Related Art
The science of gas analysis has numerous useful applications, from the measurement of human respiratory gases to the testing of pollutant levels in the atmosphere and vehicle emissions. Historically, this analysis was accomplished by collecting the gas that was to be sampled in bags and then sampling the collected gas or collecting solids or liquids off-line from the flow of the gases being analyzed. These techniques suffered from an inability to provide instantaneous dynamic information, and generally only measured a single component per technique. Additionally, the techniques were time consuming, sometimes taking weeks to perform a single analysis. Gas analysis techniques performed without liquids or solids have included chemilluminescence, flame ionization, and total hydrocarbon analysis. The gas analysis techniques performed without liquids or solids sometimes suffered from the cross-interference of added chemicals and also the inability to test for some constituent components.
More recent technological advances have made Fourier transform infrared (FTIR) spectrometric techniques available for use in gas analysis. For example, U.S. Pat. No. 4,928,015 discloses a method of using FTIR quality control techniques for analyzing multi-component constituency in gas emission flow. An FTIR spectrometer can provide simultaneous real-time concentration measurements of exhaust gas components, and is applicable for those gases that absorb infrared radiation in a sample because of the molecular oscillations and rotations. Those gases show a specific infrared absorption at different wavelengths resulting in typical spectra. All of the spectra of gases to be analyzed by the FTIR are stored in the instruments memory, and then those reference spectra are compared with the spectra of the sample gases during analysis.
One of the chief problems faced in sampling gases using FTIR analysis is obtaining a dry sample. Any moisture in the sample can significantly alter test data. As such, it is imperative that water be removed so that the moisture will not interfere with the FTIR readings. Some methods for obtaining a satisfactory sample include heating the gas itself to a temperature in excess of 100xc2x0 C. in order to maintain any water present in a vapor state, diluting the gas flow with the addition of large quantities of a non-reactive gas such as nitrogen, or passing the gas though a bank of desiccants to dehydrate it. All three of these methods have drawbacks. In the case of heating the gas to temperatures in excess of 100xc2x0 C., any reference or comparative samples also have to be heated to the same temperature to achieve an accurate comparison. Furthermore, even when maintained in a vapor state, the moisture may provide interference with certain low level FTIR measurement analysis. Diluting the gas with large quantities of a non-reactive gas decreases the level of sensitivity of the sample that can be obtained due to the dilution of the sample contents, thus providing a less accurate analysis. Also, dilution requires the presence of large tanks containing the diluting gas, making a compact system difficult to achieve. And, finally, passing the sample through a desiccant bank often removes gaseous components that are desirable for testing along with the water vapor.
The most recent technological advances in the art of moisture removal include the use of selectively permeable materials to remove water vapor via ionic channels that can absorb water molecules. For instance, U.S. Pat. No. 5,042,500 discloses a drying sample line for coupling a patient""s expiratory gases to a gas analyzer. The drying sample line comprises concentric tubes wherein the innermost tube is fabricated from a perflourinated polymer material sold as Nafion(copyright). The perflourinated polymer material exhibits high permeability to water vapor but does not readily pass other gases. The expiratory gas is drawn through the inner tube and, simultaneously, dried air is made to pass through the outer tube in a counterflow direction relative to the expiratory gases. Because of the properties of the perflourinated polymer material, water vapor (i.e., moisture) contained in the expiratory gas being coupled to the analyzer passes through the wall of the tube and into the dried air stream. Consequently, the water vapor is removed from the expiratory gas mixture being applied to the analyzer.
More particularly, the perflourinated polymer material has a Teflon backbone, with periodic side chains of perflourinated ether terminating in a sulfonic acid group. These acid groups form xe2x80x9cionic channelsxe2x80x9d that extend through the walls of the perflourinated polymer tubing. Each sulfonic acid group can absorb up to 13 molecules of water. Where the partial pressure of the water in the sample exceeds that external to the tubing, water molecules will travel along the ionic channels and be release outside the tubing. This process is selective for water, although some other species such as alcohols, ketones and ammonia may experience some loss.
Other products on the market employ in-line systems utilizing perflourinated polymer tubing for drying gas streams. However, the present products do not lower the moisture level sufficiently for accurate testing in all applications, particularly where a compact system is required because of space limitations. A more efficient process, chiefly one that is compact enough for the needs of various testing environments, is required.
In addressing the needs and deficiencies of the prior art, a water removal system for removing moisture from a gas stream is provided.
The water removal system of the present invention comprises a plurality of drying tubes coupled together in series. The drying tubes have a first flow path for a sample gas and a second flow path for a purge gas. Both the sample gas and the purge gas flow through the drying tubes, the sample gas flowing through a first flow path, and the purge gas flowing through a second flow path. At least one of the plurality of drying tubes is disposed in a cooling chamber maintained at a reduced temperature relative to the ambient temperature of the sample gas stream. A purge air source provides the purge gas. The cooling chamber reduces the temperature of the sample gas to an approximate dew point to increase the efficiency of the drying tubes.
More particularly, the drying tubes each comprise an outer shell having a sample gas inlet and a sample gas outlet, a plurality of internal tubes comprises of a selectively permeable ion exchange polymer material are disposed within the outer shell, extending between the sample gas inlet and the sample gas outlet. The sample gas flows through the internal tubes, and the purge air flows inside the outer shell along the internal tubes. Water is removed from the sample gas inside the internal tubes via ionic channels that absorb water molecules.
A more complete understanding of the system for removing water from a gaseous sample will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings that will first be described briefly.